Various devices for displaying images exist. One example is digital image projectors. Digital image projectors are widely used to project color images generated from digital signals onto a display surface. In some cases, the display surface may be the front of a reflective display screen, for example, in a theater or conference-room. In other cases, the display surface may be the rear of a semi-transparent diffusive screen of a rear-projection display monitor or projection television.
Portable digital image projectors are common. Such digital image projectors, while connected to a personal computer or other image/video signal source, typically sit on a supporting surface and are directed at a display surface on which images or video is to be shown. Many of these projectors use transmissive or reflective liquid crystal displays. Other such projectors use different imaging devices, such as digital micro-mirrors. These projectors can display images one at a time or as a sequence of images, as in the case of video.
Digital projectors are typically designed so that undistorted rectangular images are projected on the display surface when the projector is placed horizontally on a level support surface with the projector's optical axis lined up perpendicular to the display surface. However, if this alignment or orientation is not made, the resulting image on the display surface may be distorted. In many cases, the distorted image will appear as a trapezoid or an arbitrarily shaped quadrilateral. The non-rectangular shape of the resulting projected image is referred to as keystoning.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.